Isobutene for polymerisation must be more than 99% pure and can contain only traces of butene-1 and butene-2 compounds (several tens of parts per million, ppm). If the degree of impurities in the isobutene is too high, the quality of the polymers obtained is poorer and the polymerisation yield is lower. The other olefinic hydrocarbons containing 4 carbon atoms per molecule must therefore be eliminated from a hydrocarbon cut containing isobutene. Since the boiling points of butene-1 and isobutene are very close, it is not possible to separate them by distillation unless considerable means are used. The other olefinic hydrocarbons containing 4 carbon atoms can be separated from the isobutene by distillation.
The principal problem in the production of high purity isobutene is thus separating butene-1 from isobutene. A number of routes can be used to carry out this separation.
The first route consists of extracting using sulphuric acid: isobutene is selectively hydrated and then regenerated by treating the aqueous phase. If the temperature and concentration are properly controlled, this process can produce high purity isobutene. However, the yield normally does not exceed 90% as extraction is not complete and dimers and oligomers are formed, leading to the formation of toxic acid mud.
The second route consists of cracking methyl tertio-butyl ether (MTBE): isobutene is extracted from the C.sub.4 cut by reacting it with methanol to form MTBE. The MTBE is thus cracked to methanol and isobutene using an acid catalyst. The yield on recovery can be at least 96%. The isobutene produced is of high purity but it has to be freed of the dimethylether which can form during cracking.
The third possible route is dehydration of tertiary-butyl alcohol (TBA). In the previous operation, methanol can be replaced by water, leading to the production of TBA. The isobutene is then recovered by dehydrating the TBA. This route is not used in practice, mainly because TBA is closely linked to the propylene oxide market. TBA can be a by-product of propylene oxide from those processes.
The butene-1-isobutene separation problem has existed for a long time. A variety of solutions have been proposed but none is completely satisfactory in terms of efficiency of separation without the appearance of by-product(s). U.S. Pat. No. 2,403,672 describes a process for separating isobutene from an isobutene-butene-1 mixture which comprises introducing the mixture into an isomerisation zone and fractionating it, wherein the isomerisation catalyst also acts as a packing to allow distillation. This solution has the major disadvantage of not being very efficient in distillation and thus has a mediocre capacity for separating isobutene from butene-1.